1. Field of the Invention
The present invention relates to a surface acoustic wave device for use, for example, as a band-pass filter, and more particularly, to a surface acoustic wave device of which at least one of the input terminal and the output terminal is an unbalanced signal terminal.
2. Description of the Related Art
In recent years, the demand for surface acoustic wave devices for use in the RF stages of portable telephones and having a balanced-to-unbalanced function, a so-called balun function, have increased, and the use of such devices in the RF stages of GSM system portable telephones has started.
Regarding the balanced-to-unbalanced function, the difference between the amplitudes at a pair of terminals provided on the balance side (hereinafter, referred to as an amplitude balance characteristic) is required to be zero, and the difference between the phases (hereinafter, referred to as a phase balance characteristic) is required to be about 180xc2x0.
FIG. 6 is a schematic view of an example of a related art surface acoustic wave device having a balanced-to-unbalanced function.
In a surface acoustic wave device 200 shown in FIG. 6, longitudinally-coupled resonator type SAW filters 201 and 202, and 1 port type SAW resonators 203 and 204 are composed of Al electrodes on a piezoelectric substrate. The input sides of the SAW filters 201 and 202 are connected in parallel to each other and also are connected to an unbalanced signal terminal 205. The output sides of the SAW filters 201 and 202 are connected in series with the SAW resonators 203 and 204, respectively. The SAW resonators 203 and 204 are connected to balanced signal terminals 206 and 207, respectively.
In the SAW filter 201 shown in FIG. 6, IDTs 201a and 201b which are positioned on the unbalanced sides and are adjacent to reflectors have inverted-shapes with respect to IDTs 202a and 202b which are adjacent to the reflectors in the SAW filter 202. As a result, a signal output from the balanced signal terminal 206 and the signal output from the balanced signal terminal 207 have a phase difference of 180xc2x0.
According to the configuration shown in FIG. 6, of a plurality of the electrodes of the IDT 201a and the IDT 201b contained in the SAW filter 201, the electrode fingers 201d and 201e which are adjacent to the center IDT 201c are connected to the unbalanced signal terminal 205, respectively. On the other hand, of a plurality of the electrodes of the IDT 202a and the IDT 202b of the SAW filter 202, the electrode fingers 202d and 202e which are adjacent to the center IDT 202c are connected to the ground potential, respectively. Accordingly, the phase characteristic of the SAW filter 202 is not exactly equal to the inverted characteristic obtained when the phase of the SAW filter 201 is inverted by 180xc2x0. Thus, the phase balance characteristic of the surface acoustic wave 200 device can not be equal to 180xc2x0 which is an ideal value.
Moreover, in some of the related art surface acoustic wave devices having a balanced-to-unbalanced conversion function, the layout thereof such as a wiring structure ranging from an unbalanced signal terminal to a pair of balanced signal terminals is not symmetrical with respect to the unbalanced signal terminal. In this case, the parasitic capacitances of the lines extending to a pair of the balanced signal terminals become different. This readily causes deterioration of the phase balance characteristic.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having a balanced-to-unbalanced function and a superior phase balance characteristic.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate, at least one interdigital transducer IDT arranged on the piezoelectric substrate, an input terminal and an output terminal, one of the input terminal and the output terminal including an unbalanced signal terminal, the other of the input terminal and the output terminal including first and second balanced signal terminals, and a delay line provided on the unbalanced signal terminal side in the path between the unbalanced signal terminal and the first balanced signal terminal. Accordingly, the phase of a signal in the line in which the delay line is introduced is delayed, due to the introduction of the delay line. Thus, the phase balance characteristic is greatly improved. As a result, the surface acoustic wave device having a balanced-to-unbalanced function with a superior phase balance characteristic is provided.
Preferably, the delay line is disposed on the piezoelectric substrate. In this case, the surface acoustic wave device can be easily designed compared to the case where the delay line is disposed on the package. Moreover, the delay line can be formed easily with high precision on the piezoelectric substrate using a photolithography technique, for example. Moreover, the design of the delay line can be easily changed.
According to a second preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave filter element including a first piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the first piezoelectric substrate, a second surface acoustic wave filter element including a second piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the second piezoelectric substrate, the second surface acoustic wave filter element having a transmission phase characteristic that is different by about 180xc2x0 from that of the first surface acoustic wave filter element, the first and second surface acoustic wave filter elements each having an input terminal and an output terminal, certain ones of the input terminals and the output terminals of the first and second surface acoustic wave filter elements being commonly connected to each other at a common connection point, an unbalanced signal terminal being connected to the common connection point, the other terminals of the input terminals and the output terminals of the first and second surface acoustic wave filter elements defining first and second balanced signal terminals, respectively, and a delay line provided between the common connection point and the first surface acoustic wave filter element. Thus, the phase of a signal on the first surface acoustic wave filter element side is delayed compared to the phase of the signal on the other surface acoustic wave filter element side, and hence, the phase balance characteristic is greatly improved.
Also, preferably, the delay line is disposed on the piezoelectric substrate. In this case, the surface acoustic wave device can be easily designed compared to the case where the delay line is disposed on the package. Also, the cost can be greatly reduced. Moreover, the delay line can be formed easily with high precision on the piezoelectric substrate using a photolithography technique, for example. Further, the design of the delay line can be easily changed.
Preferably, the length of the signal line between the common connection point and the IDTs of the first surface acoustic wave filter element electrically connected to the common connection point is different from the length of the signal line between the common connection point and the IDTs of the second surface acoustic wave filter element electrically connected to the common connection point. This line length difference defines the delay line.
Also, preferably, the device is configured so that the phases of input-output signals in the line between the unbalanced signal terminal and the first balanced signal terminal and, those of the input-output signals in the line between the unbalanced signal terminal and the second balanced signal terminal are different by about 0.5xc2x0 to about 4xc2x0 from each other, due to the delay line.
Preferably, the surface acoustic wave device includes a second delay line which is connected to the first balanced signal terminal and is disposed on the piezoelectric substrate. In this case, both the delay lines can be dispersed for arrangement, and thereby, the phase balance characteristic can be more effectively improved. Moreover, the length of each delay line can be reduced, and the delay lines can be easily arranged.
Further, preferably, the device is configured so that the phases of input-output signals in the line between the unbalanced signal terminal and the first balanced signal terminal and, those of the input-output signals in the line between the unbalanced signal terminal and the second balanced signal terminal are different by about 0.5xc2x0 to about 4xc2x0 from each other, due to the above-described delay line and the second delay line. Thus, the phase balance characteristic of the surface acoustic wave device can be effectively improved, and moreover, the size can be further reduced, while deterioration of other characteristics can be prevented.
The above-described advantages can be obtained in the configuration of the device in which the delay line and the second delay line are arranged in such a manner that the phases of an electrical signal are different from each other by a degree of about 0.5xc2x0 to about 4xc2x0.
According to a third preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave filter element including a first piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the first piezoelectric substrate, a second surface acoustic wave filter element including a second piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the second piezoelectric substrate, the second surface acoustic wave filter element having a transmission phase characteristic that is different by about 180xc2x0 from that of the first surface acoustic wave filter element, the first and second surface acoustic wave filter elements each having an input terminal and an output terminal, selected ones of the input terminals and the output terminals of the first and second surface acoustic wave filter elements being commonly connected to each other at a common connection point, an unbalanced signal terminal being connected to the common connection point, and the other terminals of the input terminals and the output terminals of the first and second surface acoustic wave filter elements defining first and second balanced signal terminals, respectively, wherein a first length of the signal line between the common connection point and the interdigital transducers of the first surface acoustic wave filter element electrically connected to the common connection point is different from a second length of the signal line between the common connection point and the interdigital transducers of the second surface acoustic wave filter element electrically connected to the common connection point. Accordingly, the phase of one of the first and second surface acoustic wave filter elements is delayed compared to that of the other of the first and second surface acoustic wave filter elements, and thereby, the phase balance characteristic is greatly improved.
Preferably, the first length of the signal line between the common connection point and the interdigital transducers of the first surface acoustic wave filter element electrically connected to the common connection point and the second length of the signal line between the common connection point and the interdigital transducers of the second surface acoustic wave filter element electrically connected to the common connection point are different from each other such that the phases of input-output signals are different from each other by a degree of about 0.5xc2x0 to about 4xc2x0 in the line between the unbalanced signal terminal and the first balanced signal terminal and in the line between the unbalanced signal terminal and the second balanced signal terminal. In this case, the phase balance characteristic is more effectively improved, the size can be further reduced, and deterioration of other characteristics can be reliably prevented.
Preferably, on the piezoelectric substrate, the length of the signal line between the unbalanced signal terminal and the first balanced signal terminal and the length of the signal line between the unbalanced signal terminal and the second balanced signal terminal are different from each other, and the difference L (mm) between the line lengths preferably satisfies the formula 417/fc less than L less than 3330/fc in which fc represents the center frequency (MHz). Also, in this case, the phase balance characteristic of the surface acoustic wave device can be more effectively improved.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.